1. Field of the Invention
This invention pertains generally to a material test specimen for conducting stress and strain measurements and more specifically to a test specimen tailored to provide information on multiaxial stress and strain states in a controlled manner under tensile deformation.
2. Description of the Related Art
Identification of the fracture limit surface for multiaxial strain states, as shown in FIG. 1, is needed for manufacturing process design, structural design and structural integrity prediction. This is particularly true for strain-path-dependent fracture in metal alloys. Current practice in multiaxial strain state fracture limit diagrams is based on two existing methods for determining material mechanical behavior under multiaxial stress states; "tension-torsion-internal pressure" testing apparatus and the "hydrostatic bulge" testing apparatus. The material specimens are either hollow cylindrical tubes of uniform wall thickness or sheet specimens of uniform thickness, respectively. Both of these specialized test procedures are effective but require specialized test equipment which is generally specifically designed and dedicated to the purpose of multiaxial material characterization and testing. Both procedures have a low "data yield" per specimen, producing one strain history and one fracture datum point per specimen. Both test procedures also strive to preserve homogenous deformation fields for their ease of data reduction that such direct measurement allows. Determining a complete fracture limit diagram for multiple stress states requires many individual tests and is time consuming and expensive.
This invention teaches a new material test specimen geometry, wherein, the geometry features commonly referred to as "stress concentrators" generate stress and strain gradients in the material under test, supplemented by computational simulation of the test specimen.
The computational simulation of material test specimens, as a means of using nonuniform material deformation fields, has been used by K. S. Pister, Constitutive Modeling and Numerical Solution of Field Problems, Nuc. Engng. Design, Vol. 28, pp. 137-146, 1974; Idling et al., Identification of Nonlinear Elastic Solids by a Finite Element Method, Comp. Meth. Appl. Mech. Engng., Vol. 4, pp. 121-142, 1974; Norris et al., A Computer Simulation of the Tension Test, J. Mech. Phys. Solids, Vol. 26, pp. 1-19, 1978; and Matic et al., The Relationship of Tensile Specimen Size and Geometry Effects to Unique Constitutive Parameters for Ductile Materials, Proc. Royal Soc. Lond., Vol. A417, pp. 309-333, 1988; for nonlinear elastic and elastic-plastic constitutive parameter determination.